1. Field of the Invention
The present invention relates to a novel zoom lens and imaging device. More particularly, it relates to a compact, inexpensive zoom lens with a variable power ratio of about three suitable for use with a digital still camera or the like and an imaging device using the zoom lens.
2. Description of the Related Art
Recent years have witnessed a wide diffusion of imaging devices, such as digital still cameras, which are equipped with a solid-state image pickup device. As such digital still cameras equipped with a solid-state image pickup device become more popular than before, they are required to offer an increasingly high image quality. Particularly, those digital still cameras equipped with a solid-state image pickup device having a large number of pixels need an imaging lens, especially a zoom lens, with good image-forming performance. With a demand for compact devices, particularly thin ones, being strong, zoom lenses are also required to be compact with a small thickness while offering high performance. Furthermore, a demand is also high for lenses which are easy to produce.
A zoom lens which is smaller in size and higher in image forming performance than conventionally known zoom lenses is disclosed in patent document 1. The zoom lens is a three-group zoom lens including a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power arranged in the mentioned order from an object side. In the zoom lens, however, the negative lens disposed closer than the other lenses to the object has an aspherical layer made of resin. Thus, adequate cost reduction has not been made on the zoom lens.
Patent document 2 discloses another three-group zoom lens including a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power arranged in the mentioned order from an object side. In the zoom lens, however, the negative lens disposed closer than the other lenses to the object has a concave surface with a strong curvature formed with a glass lens having an aspherical surface. Producing such a glass lens involves a high degree of difficulty.
Patent document 3 discloses a still another three-group zoom lens including a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. An aspherical surface included in the first lens group of the zoom lens is formed on a positive lens, so that difficulty involved in producing a lens with an aspherical surface has been reduced. In the zoom lens, however, each of the lens groups is thick, so that miniaturizing the zoom lens in a retracted state is difficult.    [Patent document 1] Japanese Patent Laid-Open No. 2002-350726    [Patent document 2] Japanese Patent Laid-Open No. 2002-244043    [Patent document 3] Japanese Patent Laid-Open No. 2004-4765